Generally, a percussive type photoflash lamp (see FIG. 1) comprises a hermetically sealed, light-transmitting envelope containing a source of actinic light and having a primer secured thereto. Such photoflash lamps are disclosed in U.S. Pat. No. 3,439,992; U.S. Pat. No. 3, 511,586; U.S. Pat. No. 3,540,820; and U.S. Pat. No. 3,706,521.
More particularly, the percussive type photoflash lamp comprises a length of glass tubing constricted to a tip at one end (domed end) thereof and having a primer cup secured to the other end thereof. The length of glass tubing which defines a lamp envelope contains a combustible material, such as shredded zirconium foil, and a combustion-supporting gas such as oxygen. The primer cup comprises a metal shell within which an anvil and a charge of ignitable material (primer) are disposed.
Operation of a percussive-type photoflash lamp is initiated by a firing pin (also referred to as a striker) which is caused to strike the bottom of the primer cup to promote ignition of the primer disposed therein. Deflagration of the primer ignites the shredded zirconium foil, the combustion of which is the source of the actinic light.
As discussed in U.S. Pat. No. 3,439,992, because of its very rapid burning characteristics, deflagration of the primer causes a very high velocity flow of flame and hot gases up the axis of the flashlamp. This gas flow, in turn, impinges on the shredded zirconium foil, and frequently compresses the shreds into a tightly compacted ball in the domed end of the lamp envelope. Confinement of combustion in the domed end of the lamp envelope imparts a large thermal shock to the lamp envelope. This thermal shock sometimes causes the breakage of the domed end portion of the glass envelope, causing, in turn, the hot gasses to escape outwards at a high velocity and, together with flying glass shreds, to create a potential safety hazard.